Thickness control system for a rolling mill

ABSTRACT

A system for controlling strip thickness in a rolling mill in which the rolling pressure applied to material is continuously measured or the strip thickness on the output side is continuously detected as a parameter of the outgoing thickness of the rolled strip and the result of measurement or detection of the parameter is used to obtain an autocorrelation. The power spectra for the entire region and a predetermined region covered by the autocorrelation are produced, so that the ratio of roll eccentricity component to the entire variation is obtained from the ratio between the two spectra. The roll eccentricity component is applied to a feedback loop of the control system in order to eliminate the effect of the roll eccentricity component on the automatic thickness control system.

[54] THICKNESS CONTROL SYSTEM FOR A ROLLING MILL [75] Inventors: Ken Ichiryu, Mito; Masayuki Shigeta, Katsuta; Ichiro Nakamura, Katsuta; Toshiyuki Kajiwara, Hitachi, all of Japan [73] Assignee: Hitachi, Ltd., Japan [22] Filed: Oct. 15, 1974 [21] Appl. No.: 515,030

[30] Foreign Application Priority Data Oct. 17, 1973 Japan i. 48-115905 [52] US. Cl. 72/8; 72/16; 72/21 [51] Int. Cl. B21B 37/00 [58] Field of Search 72/8-12, 21, 72/16 [56] References Cited UNITED STATES PATENTS 3,100,410 8/1963 Hulls et al 72/8 3,543,549 12/1970 Howard ..72/8

Primary Examiner-Milton S. Mehr Attorney, Agent, or Firm-Craig & Antonelli [5 7] ABSTRACT A system for controlling strip thickness in a rolling mill in which the rolling pressure applied to material is continuously measured or the strip thickness on the output side is continuously detected as a parameter of the outgoing thickness of the rolled strip and the result of measurement or detection of the parameter is used to obtain an autocorrelation. The power spectra for the entire region and a predetermined region covered by the autocorrelation are produced, so that the ratio of roll eccentricity component to the entire variation is obtained from the ratio between the two spectra. The roll eccentricity component is applied to a feedback loop of the control system in order to eliminate the effect of the roll eccentricity component on the automatic thickness control system.

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FIG. 3

FIL -23 US. Patent Dec.30, 1975 Sheet3 of3 THICKNESS CONTROL SYSTEM FOR A ROLLING MILL BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thickness control system for a rolling mill, or more in particular to a thickness control method aimed at removing the influence of the roll eccentricity.

2. DESCRIPTION OF THE PRIOR ART Recently, there has been an increased demand for the accuracy of thickness of strips, and as result a gagemeter type automatic thickness control system based on the so-called BISRA-AGC (Automatic gage control developed by BISRA) has made a tremendous progress.

This gagemeter type automatic thickness control system controls values such as a thickness command hd, no-load roll gap S, rolling pressure P, and mill modulus Km so as to satisfy the following equation:

hd( S+P/Km)= Those values are essential for controlling thickness of strips in a rolling mill.

However, the gagemeter type automatic thickness control system has suffered from disadvantages in that the presence of eccentricity in respective rolls leads to the failure to maintain the roll gap constant as well as to meet the objectives of the thickness control. In other words, with the gagemeter type control system, in case the rolling pressure is increased, it is so designed that no-load roll gap S be decreased on the assumption that the increase in the rolling pressure has been caused by the increase in thickness of a strip on the input side. However, in case the roll gap is decreased due to roll eccentricity, the rolling pressure will be increased, so that contrary to the need to increase the roll gap, the aforesaid control system will function so as to decrease the roll gap. Accordingly, it is an important problem imposed on the gagemeter type automatic thickness control system to remove or avoid the influence of roll eccentricity.

Meanwhile, many attempts have so far been made to overcome this problem. However, most of these attempts have failed to meet an intended success, because of too complicated construction or the failure to obtain desired accuracy, with the result of resorting to the skill of operators to solve this problem. For instance, the simplest attempt among the above-referred attempts from the viewpoint of control is to modify the automatic gage control system such that means is provided to make periodically ineffective the feedback of the rolling pressure component by connecting, in the feedback loop of the control system, a filter which is adapted to allow the rolling pressure component to pass in synchronism with the roll eccentricity cycle fe. In this attempt, as well, the following disadvantages are encountered:

I. Since the component of variation of the roll pressure is excluded from the gage control system during the time depending on the roll eccentricity each cycle of the roll rotation, the component of thickness variation due to variation of the thickness at the input side of the mill is also excluded from the gage control system, resulting in incapability of controlling the component of thickness variation which must be principally controlled.

2. The so-called resonance type filter adapted to tune with a roll eccentricity frequency fe has a given band width (sensitive zone width), thereby presenting shortcoming of passing signals whose frequencies are close to that of the signal intended to be passed therethrough.

Another attempt is that the generating position of roll eccentricity and the frequency thereof are first assumed, and then the thickness is controlled according to the aforesaid assumption. In other words, it is assumed that the roll eccentricity arises at a backup roll, while the frequency of roll eccentricity be fe, and that a roll eccentricity frequency component which has been detected be roll eccentricity alone. In this case, the wave form is Fourier-analyzed to take out the roll eccentricity frequency component alone, thereby compensating for the roll eccentricity component in the thickness control system.

However, the disturbance in the roll system occurs in a work roll section, as well, presenting a complicated pattern, including its frequency component. For this reason, the desired accuracy can not be obtained merely by detecting and compensating for the eccentricity component of a single roll according to the aforesaid system.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a system for thickness control in which the effect of the roll eccentricity component on the thickness control system of gagemeter type is removed by accurately identifying the roll eccentricity component, thereby achieving a highly accurate thickness control of a rolling mill.

In order to achieve the above-mentioned object, according to the invention, a parameter relating to the thickness on the output side is continuously measured and the result of the measurement is used to detect external disturbance due to the roll eccentricity by statistical techniques based on the correlation, thus eliminating the external disturbance from the thickness control system. Specifically, the present invention is characterized in that the autocorrelation function R P of the thickness parameter on the output side is obtained, so that the roll eccentricity component is taken out of the all variation components by obtaining the power spectra 4),, and 4),, respectively from the entire region and a predetermined region associated with the auto correlation function R,,. The roll eccentricity component thus taken out is applied as a command to the automatic thickness control.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic diagram showing a typical automatic thickness control system of gagemeter type according to the present invention.

FIG. 2 shows a block diagram of a circuit for detecting the roll eccentricity by the method of thickness control according to the invention.

FIG. 3 is a diagram showing an example of the autocorrelation function of rolling pressure.

FIG. 4 is a block diagram showing a flow chart for calculating the phase relation of the roll eccentricity against the rotation of the reinforcing roll on the basis of rolling pressure.

FIG. 5 is a block diagram showing an automatic thickness control system of gagemeter type according to an embodiment of the invention.

3 FIG. 6 is a block diagram showing a flow chart for calculating the phase relation of the roll eccentricity on the basis of thickness variation on the output side.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, there is shown a diagram of a gagemeter type automatic thickness control system. As shown in this figure, a rolling mill consists of a work roll 2 adapted to directly roll a strip 1, and a backup roll 3 externally supporting the work roll 2. The roll screw-down operation of the rolling mill is accomplished by means of a hydraulic jack provided at the ends of the lefthand and righthand rolls. The hydraulic jack consists of a hydraulic cylinder 5 and a ram 6 and is so designed as to adjust the roll gap by adjusting the amount of oil within the hydraulic jack with the aid of a servo-valve 4. For the thickness control at the time of rolling, the displacement S of the ram 6 is measured by means of a displacement meter 7, and then a measurement thus obtained is negatively fed back to be compared with the thickness command hd. On the other hand, rolling pressure P is measured by a pressure gage 8, and then the measurement value is divided by a mill constant Km at a coefficient multiplier 9 and then the value thus obtained is applied to the summing point 10 to be negatively fed back to the thickness command hd. Thus, the aforesaid respective values are controlled so as to meet the relationship, hd (S P/Km) 0, thereby maintaining the thickness of the rolled strip constant.

The fact that the roll eccentricity exerts the important influence on the gagemeter type thickness control system has been referred to under the heading ofTHE DESCRIPTION OF THE PRIOR ART". According to the present invention, a statistical technique is used to find roll eccentricity, thereby removing this factor from the thickness control system. Now, the fundamental principle of the present invention will be described below.

It is known that the thickness variation of the output side is generally attributable to the thickness variation on the input side and roll eccentricity. The thickness variation on the input side is an irregular one in most cases given by statistically random signal whereas the roll eccentricity is subjected to periodical variation. In order to take out the roll eccentricity component, therefore, it is necessary to separate regular components from irregular components by statistical data processing.

According to the invention, the rolling pressure which is closely related to the thickness variation on the output side and easily measured is continuously detected and the autocorrelation function R (1') of the result P(t) of detection is obtained. Such an autocorrelation function R is defined as where r is a delay time. As shown in FIG. 3, this autocorrelation function R ('r) varies with the delay time 1'. Where the delay time 1- is close to zero, that is O 5 1' 7R1 or 0 g 17 7R in the drawing, for example, the correlation function R indicates the irregularity of the rolling pressure generally corresponding to the irregular components of the thickness variation on the input side. As a result, the frequency component based on the roll eccentricity presents itself in a large delay time, thus making it necessary to eliminate the portions represented by 0 'r 7R or 0 1 TR2.. In other words, on the basis of the correlation function Rp in the new range 1' g 1' g 1', having an origin at the delay time 1' (1-,, in FIG. 3) corresponding to an appropriate point beyond the first 0 point or second 0 point on the-correlation curve, a power spectrum 4) defined by the following equation is obtained:

indicates the ratio of the pure roll eccentricity power to the entire power at roll eccentricity cyclefe. According to the present invention, only the roll eccentricity component separated on the above-mentioned theory is produced as an output from the automatic thickness control system of gagemeter type thereby to eliminate the influence thereof.

This is, however, not more than a successful construction of automatic thickness control system without any influence of the roll eccentricity but the effect of the roll eccentricity continues to be present in the thickness variation on the output side. If the thickness is to be controlled at a constant level, therefore, it is necessary to eliminate the roll eccentricity more positively. The phase relation of the roll eccentricity as against a signal synchronous with the rotation of a roll, especially the reinforcing roll which is most influential to the roll eccentricity is obtained, and then according to such a phase relation, instructions are given to the automatic control system to offset the roll eccentricity. What is important for the phase relation of roll eccentricity are the gain I e of the roll eccentricity component and the phase angle e. These two factors may be obtained either directly from the rolling pressure or from the thickness variation on the output side. Thus, by obtaining the gain and phase angle, it is possible to apply an input to the control system in such a manner as to offset the influence thereof. In this case, however, to know the transfer function between the input and the response by the roll section in advance is a prerequisite, which may be accurately met by a frequency response method for the hydraulic reduction control system.

An actual construction of the invention will be described below with reference to the accompanying drawings.

Referring to FIG. 2 showing a block diagram for calculating a component corresponding to the roll eccentricity from the rolling pressure detector circuit, the rolling mill used is the same as that shown in FIG. 1.

First, the rolling pressure is detected bya rolling pressure gage 8, and the detection signal P(t) is produced in the form of P(t 1') through a delay circuit 12.

Further, P(t) is multiplied by P(t 1') at a multiplier circuit 13, the output of which is applied to an averaging circuit 14 whereby the autocorrelation function of the rolling pressure R,D is obtained. This autocorrelation function R is divided into two. One of the results of the division is applied to a phase shifter 15 having the range T,, 7' 5 T the output of which is multiplied by e 11 r in the multiplier circuit 16 and averaged by the averaging circuit 17 thereby to obtain a power spectrum corresponding to the roll eccentricity.

The other output of the correlation function R is multipled by e 11' r in the multiplier circuit 18 for the range 0 7 7' 1 without any special process, and the result of the multiplication is integrated by the averaging circuit 19, thereby producing the total power 4),. of the rolling pressure at the eccentricity frequency fe. Also, p and are applied to the divider circuit 20 thereby to obtain B (p 4 Reference is made to FIG. 4 showing a block diagram for calculating the gain |e| of roll eccentricity and phase angle e for a signal synchronous with the rotation of the reinforcing roll 3. In the figure, the rolling pressure is detected by the rolling pressure gage 8, the output of which is applied to a rolling pressure measuring circuit 22. The output of the circuit 22 is applied to a filter 23 synchronized with the roll eccentricity frequencyfe and having such a weight as to pass the roll eccentricity component alone, whereby the output of the circuit 22 is compared with the output signal from a reference signal generator 21. The result of comparison as well as the relation between the rolling pressure and roll eccentricity is used to calculate the gain |e| and phase angle e by the eccentricity calculating circuit 24. In view of the fact that the transfer function between input and roll displacement in the servo-system of the gagemeter type can be known in advance by the frequency response method, the gain |e'| of the roll eccentricity compensating command e and the phase angle e are obtained by the circuit 25.

The block diagram of FIG. 5 shows an example of the construction of the automatic thickness control system of gagemeter type making use of the power spectrum ratio B and the roll eccentricity compensating command e' derived from the foregoing calculation processes. In the figure, the symbol hd shows a roll gap command, and an electrical signal representing the roll gap command hd is amplified by the amplifier 30, so that the output current of the amplifier is used to energize the servo-valve 4, thereby determining the roll gap S at the time of no-load through the block of the pipe 32.

The symbol 2, on the other hand, shows the amount of roll eccentricity, h the thickness on the output side, and h the thickness on input side. The difference between h and h causes a rolling pressure as shown in the block 33, and the roll gap is increased by the amount obtained by dividing the difference by the mill constant Km. Reference numeral 9 shows a block representing the reciprocal of the mill constant. A rolling pressure feedback loop 38 is formed for the purpose of dampening the variation in roll gap due to such a rolling load. The block shows a section for regulating the mill constant a, which is unity at infinite rigidity (in thecase of ordinary BISRA-AGC) and zero at natural state, its normal value employed being close to 1. The output of the block 35 is separated into a couple of loops including the lines 39 and 40. The signal on the line 40 is applied through a narrow band filter 36, which has no loss nor phase delay only at the roll-eccentricity frequency, through the eccentricity regulating section 37 with the coefficient of ,B, and then combined with the signal from the line39 so that the output 41 of the regulator section 37 is subtracted from the line 39 and the result of subtraction is fed back to the input terminal 42. In this way, only the portion corresponding to the roll eccentricity is eliminated, with the result that the thickness component on the input side alone is fed back, thus making up an normal automatic thickness control system of gagemeter type;

Further, according to the invention, since the phase relation of pure roll eccentricity e is given from FIG. 4, the application to the input terminal 42 the phase relation e for offsetting it permits the roll eccentricity to be completely eliminated, thus making it possible to regulate the thickness on the output side exactly at a fixed level.

By the way, the roll eccentricity 2 obtained from the block diagram of FIG. 4 is obtained through the calculation processes by measuring the strip thickness on the output side as shown in FIG. 6. The thickness of the strip rolled on the output side is continuously detected by the X-ray thickness gages 26 and 27 and produced from a thickness measuring circuit 22. This signal is compared with the signal 21 synchronous with roll rotation at the filter 23, and the result of comparison is applied to the eccentricity calculation circuit 24 thereby to obtain the gain [2 of roll eccentricity and the phase angle e. The phase relation of roll eccentricity is applied to an input calculation circuit 25 whereby the offset signal 2' is obtained.

In spite of the above-described embodiments wherein the roll eccentricity component is obtained by continuously'measuring the rolling pressure, it will be easily understood that the fundamental principle of the invention permits the roll eccentricity to be calculated also on the basis of autocorrelation of the thickness on the output side by measuring the same.

The foregoing construction ofthe thickness control according to the inventionmakes it possible to separate the pure roll eccentricity alone, resulting in a highly accurate thickness control whereby roll eccentricity component is capable of being completely eliminated from the automatic thickness control system of gagemeter type in common use.

In addition, according to the invention, a roll eccentricity compensation command enables a rolling operation while at the same time compensating for any roll eccentricity.

It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efficiently attained, and since certain changes may be made in the above construction without departing from the spirit and scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

We claim:

'1. In a strip thickness control system of the gagemeter type comprising a plurality of rolls, a hydraulic jack for giving rolling power to said rolls, a flow rate control valve for adjusting the roll gap by controlling the amount of oil in said hydraulic jack. a valve control device, means for giving a command associated with a desired thickness to said control device and a position detector for detecting the roll gap, the output of said position detector being fed back to said valve control device; a method of controlling the strip thickness for the rolling mill comprising steps of continuously detecting a value of at least one parameter relating to the outgoing thickness of a strip rolled by said rolls, producing an autocorrelation function R of the detected value of said parameter, producing a power spectra for the whole region and a power spectra for a predetermined part of the region covered by said autocorrelation function, determining a roll eccentricity power from the ratio between said power spectra for the predetermined part of the region and said power spectra for the whole region, and applying said roll eccentricity power to the strip thickness control system so as to compensate for the variation of the outgoing thickness of the strip due to the roll eccentricity.

2. A method for controlling strip thickness according to claim 1, in which the rolling pressure is continuously detected as a parameter relating to the outgoing strip thickness.

3. A method for controlling strip thickness according to claim 1, in which the power spectra over the whole region of the entire frequency variation corresponding to roll eccentricity is calculated by the use of the whole positive region of a delay time, whereas the power spectra for the predetermined part of the region is calculated by the use of regions excepting those regions close to the delay time of zero.

4. A method for controlling strip thickness according to claim 1, further comprising inserting a narrow band filter in said feedback loop of said thickness control system for passing only those components close to the roll eccentricity frequency in synchronism with said roll eccentricity frequency and the output of said filter is rendered proportional to the ratio of power spectra of the predetermined part of the region to power spectra of the whole region.

5. A method for controlling strip thickness according to claim 1, further comprising determining the gain and phase angle of the roll eccentricity component in synchronism with the rotation of said rolls by detecting at least one of the strip thickness on the output side and the rolling pressure, and applying a roll eccentricity compensating command based on said gain and said phase angle to said automatic thickness control system.

6. A method for controlling strip thickness according to claim 2, in which the power spectra over the whole region of the entire frequency variation corresponding to roll eccentricity is calculated by the use of the whole positive region of a delay time, whereas the power spectra for the predetermined part of the region is calculated by the use of regions excepting those regions close to the delay time of zero.

7. A method for controlling strip thickness according to claim 2, further comprising inserting a narrow band filter in said feedback loop of said thickness control system for passing only those components close to the roll eccentricity frequency in synchronism with said roll eccentricity frequency and the output of said filter is rendered proportional to the ratio of power spectra of the predetermined part of the region to power spectra of the whole region.

8. A method for controlling strip thickness according to claim 2, further comprising determining the gain and phase angle of the roll eccentricity component in synchronism with the rotation of said rolls by detecting at least one of the strip thickness on the output side and the rolling pressure, and applying a roll eccentricity compensating command based on said gain and said phase angle to said automatic thickness control system.

9. A method for controlling strip thickness according to claim 1, in which the outgoing thickness of the rolled strip is continuously detected as a parameter relating to the outgoing strip thickness.

10. In a strip thickness control system of the gagemeter type comprising a plurality of rolls, a hydraulic jack for giving rolling power to said rolls, a flow rate control valve for adjusting the roll gap by controlling the amount of fluid in said hydraulic jack, a valve control device, means for giving a command associated with a desired thickness to said control device and a position detector for detecting the roll gap, the output of said position detector being fed back to said valve control device, the improvement comprising means for generating a roll eccentricity signal and for applying said roll eccentricity signal to the strip thickness control system so as to compensate for the variation of the outgoing thickness of the strip due to the roll eccentricity, said roll eccentricity signal generating means including means for continuously detecting a value of at least one parameter relating to the outgoing thickness of a strip rolled by said rolls, means for producing an autocorrelation function R of the detected value of said parameter, means for producing a power spectra for the whole region and a power spectra for a predetermined part of the region covered by said autocorrelation function, means for determining a roll eccentricity power signal from the ratio between the power spectra for the predetermined part of the region and the power spectra for the whole region, and means for applying the roll eccentricity power signal to the strip thickness control system.

11. A strip thickness control system according to claim 10, wherein said means for continuously detecting a value of at least one parameter relating to the outgoing strip thickness includes means for continuously detecting the rolling pressure and providing an output signal indicative thereof, and further comprising means for providing an output signal indicative of the rotation of said rolls and determining means responsive to the roll pressure signal and the rotation signal for determining the gain and phase angle of the roll eccentricity component in synchronism with the rotation of the rolls.

12. A strip thickness control system according to claim 10, wherein the means for continuously detecting a value of at least one parameter relating to the outgoing strip thickness includes means for continuously detecting the outgoing strip thickness and providing an output signal indicative thereof, and further comprising means for providing a signal indicative of the rotation of said rolls, and determining means responsive to the outgoing strip thickness signal and the roll rotation signal for determining the gain and phase angle of the roll eccentricity component in synchronism with the rotation of the rolls. 

1. In a strip thickness control system of the gagemeter type comprising a plurality of rolls, a hydraulic jack for giving rolling power to said rolls, a flow rate control valve for adjusting the roll gap by controlling the amount of oil in said hydraulic jack, a valve control device, means for giving a command associated with a desired thickness to said control device and a position detector for detecting the roll gap, the output of said position detector being fed back to said valve control device; a method of controlling the strip thickness for the rolling mill comprising steps of continuously detecting a value of at least one parameter relating to the outgoing thickness of a strip rolled by said rolls, producing an autocorrelation function R of the detected value of said parameter, producing a power spectra for the whole region and a power spectra for a predetermined part of the region covered by said autocorrelation function, determining a roll eccentricity power from the ratio between said power spectra for the predetermined part of the region and said power spectra for the whole region, and applying said roll eccentricity power to the strip thickness control system so as to compensate for the variation of the outgoing thickness of the strip due to the roll eccentricity.
 2. A method for controlling strip thickness according to claim 1, in which the rolling pressure is continuously detected as a parameter relating to the outgoing strip thickness.
 3. A method for controlling strip thickness according to claim 1, in which the power spectra over the whole region of the entire frequency variation corresponding to roll eccentricity is calculated by the use of the whole positive region of a delay time, whereas the power spectra for the predetermined part of The region is calculated by the use of regions excepting those regions close to the delay time of zero.
 4. A method for controlling strip thickness according to claim 1, further comprising inserting a narrow band filter in said feedback loop of said thickness control system for passing only those components close to the roll eccentricity frequency in synchronism with said roll eccentricity frequency and the output of said filter is rendered proportional to the ratio of power spectra of the predetermined part of the region to power spectra of the whole region.
 5. A method for controlling strip thickness according to claim 1, further comprising determining the gain and phase angle of the roll eccentricity component in synchronism with the rotation of said rolls by detecting at least one of the strip thickness on the output side and the rolling pressure, and applying a roll eccentricity compensating command based on said gain and said phase angle to said automatic thickness control system.
 6. A method for controlling strip thickness according to claim 2, in which the power spectra over the whole region of the entire frequency variation corresponding to roll eccentricity is calculated by the use of the whole positive region of a delay time, whereas the power spectra for the predetermined part of the region is calculated by the use of regions excepting those regions close to the delay time of zero.
 7. A method for controlling strip thickness according to claim 2, further comprising inserting a narrow band filter in said feedback loop of said thickness control system for passing only those components close to the roll eccentricity frequency in synchronism with said roll eccentricity frequency and the output of said filter is rendered proportional to the ratio of power spectra of the predetermined part of the region to power spectra of the whole region.
 8. A method for controlling strip thickness according to claim 2, further comprising determining the gain and phase angle of the roll eccentricity component in synchronism with the rotation of said rolls by detecting at least one of the strip thickness on the output side and the rolling pressure, and applying a roll eccentricity compensating command based on said gain and said phase angle to said automatic thickness control system.
 9. A method for controlling strip thickness according to claim 1, in which the outgoing thickness of the rolled strip is continuously detected as a parameter relating to the outgoing strip thickness.
 10. In a strip thickness control system of the gagemeter type comprising a plurality of rolls, a hydraulic jack for giving rolling power to said rolls, a flow rate control valve for adjusting the roll gap by controlling the amount of fluid in said hydraulic jack, a valve control device, means for giving a command associated with a desired thickness to said control device and a position detector for detecting the roll gap, the output of said position detector being fed back to said valve control device, the improvement comprising means for generating a roll eccentricity signal and for applying said roll eccentricity signal to the strip thickness control system so as to compensate for the variation of the outgoing thickness of the strip due to the roll eccentricity, said roll eccentricity signal generating means including means for continuously detecting a value of at least one parameter relating to the outgoing thickness of a strip rolled by said rolls, means for producing an autocorrelation function R of the detected value of said parameter, means for producing a power spectra for the whole region and a power spectra for a predetermined part of the region covered by said autocorrelation function, means for determining a roll eccentricity power signal from the ratio between the power spectra for the predetermined part of the region and the power spectra for the whole region, and means for applying the roll eccentricity power signal to the strip thickness control system.
 11. A strip thIckness control system according to claim 10, wherein said means for continuously detecting a value of at least one parameter relating to the outgoing strip thickness includes means for continuously detecting the rolling pressure and providing an output signal indicative thereof, and further comprising means for providing an output signal indicative of the rotation of said rolls and determining means responsive to the roll pressure signal and the rotation signal for determining the gain and phase angle of the roll eccentricity component in synchronism with the rotation of the rolls.
 12. A strip thickness control system according to claim 10, wherein the means for continuously detecting a value of at least one parameter relating to the outgoing strip thickness includes means for continuously detecting the outgoing strip thickness and providing an output signal indicative thereof, and further comprising means for providing a signal indicative of the rotation of said rolls, and determining means responsive to the outgoing strip thickness signal and the roll rotation signal for determining the gain and phase angle of the roll eccentricity component in synchronism with the rotation of the rolls. 